Engine emission regulations have become increasingly stringent and more widely applicable. For example, recent regulations implemented by the California Air Resources Board have required auxiliary power units provided on 2007 or newer trucks to comply with the level 3 emissions standard. The level 3 emissions standard requires an 85 percent reduction in particulate material as compared to baseline emissions generated by the source of exhaust emissions.
Engine exhaust filters suitable for removing particulate material from an exhaust stream can have a variety of constructions. One type of exhaust filter includes a cellular ceramic core defining a honeycomb of channels having plugged ends. Filters having this construction are disclosed in U.S. Pat. Nos. 4,276,071 and 4,851,015. Other exhaust filters include a filter media defined by a plug of wire mesh. Filters having this construction are disclosed in U.S. Pat. Nos. 3,499,269 and 4,902,487.
While existing exhaust filters can be effective at removing particulate material from diesel engine exhaust, such filters have a tendency to become over loaded with particulate material overtime thereby causing excessive amounts of backpressure to be generated behind the filters. To overcome this problem, different types of regeneration systems have been developed. Some regeneration systems rely on heat present in the exhaust gas from the engine to combust particulate material accumulated on the exhaust filter. However, such systems are not ideal for smaller diesel engines of the type typically used for auxiliary power units because such diesel engines generally do not emit exhaust having a high enough temperature to reliably regenerate an exhaust filter. Other systems include heaters (e.g., gas burners or electrical resistance type heaters) the provide sufficient heat to actively regenerate an exhaust filter. However, such systems can be difficult to control and are often quite expensive. Therefore, improvements are needed in exhaust filter regeneration systems.